TA30 NAD QUESTION

Loving my TA15 Head this past year and received today used TA30 which I thought would be better for outside festivals etc. Sounds good but volume level seems very low and TA30 played at 15w mode is 10db lower than TA15 in 15 w mode ( 12 O'Clock settings all controls in Top Boost mode/channel) through same cab open back EVM open back. Might this be a simple tube issue? No distortion, just not nearly the volume/power expected as compared to TA15.